Noise reducing means for cabin aircraft



Nov 2l, 1939. s. J. ZAND 3 NOISE REDUCING MEANS FOR CABIN AIRCIFUUF I Original Filed March 29, 1935 5 INVENTOR Q' TEPHEM J Znnm Patented Nov. 21, 1939 UNITED STATES PATENT OFFICE NOISE REDUCING MEANS FOR CABIN AIRCRAFT Stephen J. Zand, Forest Hills, N. Y., assignor to Sperry Gyroscope Company, Inc., Brooklyn, N. Y., a corporation of New York Original application March 29, 1933, Serial No. 663,302, now Patent No. 2,122,447, dated July Divided and this application October 27, 1937, Serial No. 1.71375 4 Claims. (Cl. 181-36) The principal object of the present invention is to reduce the noise emitted from the aircraft engines, the invention being especially adapted for reducing the noise of the engines of multi-motored craft without decreasing the efficiency thereof. In such craft it is found that several engines usually have very close periods of vibration so that the predominant sound from each is a note or less apart. Such a combination produces very disagreeable beat notes, or in other words, discords. To eliminate this, I provide means for tuning the predominant notes of the engines so they they are the same or some non-beat producing relative frequency. By this means the dominant notes or-sound periods emitted from the two engines are made to harmonize. In addition, by interference effects and proper filtering the total sound may be greatly reduced.

Referring to the drawing,

Fig. 1 is a plan view, partly in section, of a cabin airplane'employing the structure of the present invention.

Fig. 2 is a fragmentary, part sectional view illustrating the details of one form of the invention.

Fig. 3 is a fragmentary sectional view illustrating details of a slightly modified form of the invention.

Similar characters of reference are used in all of the above figures to indicate corresponding parts.

Referring, now, to the said drawing, the reference numeral I designates an aircraft, such as .an airplane, having dual engines 24 and 25, al-

wardly therefrom and having muiilers I6 and i8 thereon. I

I find as the result of considerable experimentation that it is possible to reduce the disagreeable beat notes of multi-engined planes by a method of turning or synchronizing the predominant sound frequencies caused by the engine exhaust.

' To this end, I may provide at least one of the exhaust pipes i5 or I5 (see Fig. 3) with variable end or nozzle plates 22 having different size holes 23 by which the predominant note of the exhaust may be varied so as to properly harmonize with the predominant note of theother engine and, therefore, avoid the objectionable beat notes referred to.

Another method I have shown (see Figs. 1 and 2) is to provide across pipe 2| connecting the exhaust pipes l5 and ii of the two engines. Such a pipe is preferably provided with a small opening 26 to the atmosphere which may have a short pipe 21 connected thereto, the opening of which is adjustable by a needle valve 28. By such a means the disagreeable beat notes above referred to may be eliminated either by interference or filtering, or both, and the general sound level of the engines greatly reduced.

As many changes could bemade in the 'abov construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawengines and exhaust passages therefor, of a cross pipe connecting said passages to harmonize and attenuate the predominant sound emissions from each.

2. As a means for reducing noise in multi-motor aircraft, the combination with a plurality of engines and exhaust passages therefor, of a cross pipe connecting said passages, and an adjustable tuning or filtering opening in said pipe to harmonize and attenuate the predominant sound emission from each.

3. In an aircraft, a plurality of internal combustion engines each having an exhaust passage and operated at substantially the same speeds, and means comprising a direct gas connection between the exhaust passages of said engines subjected only to the pressures caused by the engine exhaust gases operating against the friction of the exhaust conduit for eliminating beat notes produced by the simultaneous operation of said engines.

4. In an aircraft, two internal combustion mo=- tors for propelling the aircraft, each of said mo tors having an exhaust passage and an exhaust pipe connected to the exhaust passage, and a direct equalizing connection between said exhaust pipes subjected only to the pressures caused by the engine exhaust gases operating against the friction of the exhaust passage for harmonizing and attenuating the predominant sound emissions from the said motors.

STEPHEN J. ZAND. 

